As semiconductor manufacturing advances to ultra-large scale integration (ULSI), the devices on semiconductor wafers shrink to sub-micron dimension and the circuit density increases to several million transistors per die. In order to accomplish this high device packing density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines and the surface geometry such as corners and edges, of various features.
The requirement of small feature sizes with close spacing between adjacent features requires high resolution photolithographic processes. In general, photolithography utilizes a beam of light, such as U.V. waves, to transfer a pattern from a photolithographic reticle or mask onto a photoresist coating through an imaging lens. (As used herein, the term "reticle" and "mask" are interchangeable.) The mask includes opaque and transparent regions such that the shapes match those of the openings in the resist coating in the desired or predetermined pattern.
One technique currently being investigated for improving the resolution of the photolithographic process is known as phase shift lithography. With phase shift lithography the interference of light rays is used to overcome diffraction and improve the resolution and depth of optical images projected onto a target. In phase shift lithography, the phase of an exposure light at the object is controlled such that adjacent bright areas are formed preferably 180.degree. out of phase with one another. Dark regions are thus produced between the bright areas by destructive interference even when diffraction would otherwise cause these areas to be lit. This technique improves total resolution at the object and allows resolutions as fine as 0.15 .mu.m to occur.
An early patent in this field, U.S. Pat. No. 4,360,586 to Flanders et. al., was issued on Nov. 23, 1982 and assigned to MIT. This patent was directed to exposing periodic optical features on an object surface. The features were characterized by a spatial period p. According to the invention, a source of radiant energy of wavelength .lambda. illuminates a surface to be exposed through a mask having a spatial period separated from the surface by a distance approximately S.sub.n =p.sup.2 /n.lambda., where n is an integer greater than one.
With respect to semiconductor fabrication numerous laboratory techniques have been proposed to employ phase shifting in the photopatterning of semiconductor wafers. Most of the work in this area has centered around either "Alternating Phase Shifting", "Subresolution Phase Shifting", or "Rim Phase Shifting" experiments. In general, in each of these techniques a phase shift mask or reticle is constructed in repetitive patterns of three distinct layers of material. An opaque layer on the mask provides areas that allow no light transmission therethrough, a transparent layer provides areas which allow close to 100% of light to pass through, and a phase shifter layer provides areas which allow close to 100% of light to pass through but phase shifted 180.degree. from the light passing through the transparent areas. The transparent areas and phase shifting areas are situated such that light rays diffracted through each area are canceled out in a darkened area therebetween. This creates the pattern of dark and bright areas which can be used to clearly delineate features of a pattern on a photopatterned wafer.
"Alternating Phase Shifting" as disclosed in [1] is a spatial frequency reduction concept similar to the method disclosed in the Flanders et. al. patent. It is characterized by a pattern of features alternately covered by a phase shifting layer. "Subresolution Phase Shifting" [2] promotes edge intensity cut off by placing a subresolution feature adjacent to a primary image and covering it with a phase shifting layer. "Rim Phase Shifting" [3] overhangs a phase shifter over a Chrome mask pattern.
Another type of phase shift mask or reticle is known in the art as a chromeless phase shift mask. A chromeless phase shift mask is shown in FIG. 1A. With a chromeless phase shift mask 10, phase shifters 12 are formed on a transparent substrate 14 in a desired repetitive pattern. Each phase shifter 12 is formed of a transparent material, such as silicon dioxide (SiO.sub.2). The phase shifters 12 are formed with a thickness "t" that is selected to achieve a 180.degree. (.lambda.) phase shift for light passed through a phase shifter 12 in relation to light passed through the transparent substrate 14. This optimal thickness "t" can be determined by the formula: ##EQU1## where t=thickness of phase shift material
i=an odd integer PA1 .lambda.=wavelength of exposure light PA1 n=refractive index of phase shift material at the exposure wavelength
Both the thickness "t" and refractive index "n" of the phase shifter material are selected to achieve an optimum 180.degree. phase shift.
As shown in FIG. 1B, the electric field on the mask 10 is stepped 180.degree. (.pi.) out of phase by each phase shifter 12. As shown in FIG. 1C, the electric field on the wafer has a sinusoidal distribution, varying in amplitude from 1 to -1, As shown in FIG. 1D, the intensity at the wafer also has a sinusoidal distribution with a pattern of zero intensities aligned with the edges 16 of the phase shifters 12. This intensity distribution at the wafer is caused by the phase canceling of light passed through the transparent substrate 14 versus light passed through the phase shifters 16. The maximum phase canceling occurs at the edges 16 of the phase shifters 12 which correspond to areas of zero intensity at the wafer.
A pattern at the wafer 18 is shown in FIG. 1E. A pattern of darkened areas 20 formed on the wafer corresponds to maximum destructive phase canceling occurring at the edges 16 of the phase shifters.
A prior art process for forming a chromeless phase shift mask is shown in FIGS. 2A-2D. As an initial step (2A), a Quartz substrate 22 has a layer of an opaque material, such as Chrome, 24 deposited thereon. A photosensitive layer of resist 26 is next deposited upon the Chrome layer 24. The resist 26 is then patterned, and the Chrome layer is etched to form the patterned Chrome layer 28 on the substrate 22 as shown in FIG. 2B. Next, as shown in FIG. 2C, the areas 30 of the Quartz substrate 22 corresponding to openings formed in the patterned Chrome layer 28 are etched to a depth of "t". The patterned Chrome layer 28 is then stripped off of the substrate 22 to form the finished phase shift mask 32 or reticle. The phase shift mask 32 is characterized by a pattern of phase shifters 34 which are thicker than light transmissive areas 36 on the substrate 22 by a distance of "t". The distance "t" along with the index of refraction of the substrate 22 are selected to achieve a 180.degree. phase shift.
A problem with this prior art technique for forming a chromeless phase shift reticle 32 is that the light transmissive areas 36 of the phase shift mask 32 between the phase shifters 34 are roughened by the etching step (FIG. 2C) required to etch the substrate 22 to a depth of "t". In general, the phase shifters 34 must be formed with parallel and vertical (i.e., vertical to the substrate) sidewalls. This requires an anisotropic dry etch. Such an anisotropic dry etch is typically accomplished with a plasma planar system. In such a dry etching system, wafers are placed on a planar surface under an RF electrode in a plasma field. Etching ions, such as those of chlorine or fluorine, are then directed over the exposed areas of the wafer surface to be etched. Such a dry etching process forms a roughened and optically irregular substrate surface in the light transmissive areas 36 of the substrate 22 which are attacked by the etchant. The effectiveness of the finished phase shift mask 32 may be affected by the roughened surface of these light transmissive areas 36. As an example, these roughened light transmissive areas 36 may not be totally transparent and may reflect a portion of the incident light. Additionally, an index of refraction of a roughened surface may be different than an index of refraction of a smooth surface.
The present invention is directed to a process for forming a phase shift mask in which all surfaces of the mask are polished and more optically perfect.